In a mechanical ventilator for providing ventilatory (respiratory) assistance to a patient, different kinds of sources may be used for providing an inspiratory gas mixture to the patient. For example, pressurized air and/or oxygen can be used. Alternatively, a blower can be used. Unlike a pressurized air or oxygen source, the blower has a low pneumatic impedance or resistance when the flow delivery is interrupted. This may typically happen during the patient's expiration. The consequence of the low impedance is that backflow into the ventilator can occur, which can result in rebreathing and airway plateau-pressure drop. The traditional solution to this problem is to design a check-valve (or non-return valve) into the pneumatic system.
In existing systems, controlling the flow generated with a radial blower is difficult, for example it is difficult to control flow for low flow settings, for example for restrictive patients, small children, or any other low compliance patients, or for obstructive patients with high airway resistance. Radial blowers tend to behave oscillatory in case the flow level is controlled, because they may behave as ideal pressure sources at low flow situations. Traditionally this problem is addressed by adding a control valve that acts like a variable resistance, with the radial blower at an almost fixed pressure setting. However, such a valve is rather big because the flow resistance can not be too high relative to the maximum achievable blower pressure. And the power consumption of the blower increases together with the thermal load and noise production.
U.S. Pat. No. 6,722,359 B2 discloses a system with a pneumatic membrane inhalation valve. The front side of the membrane faces an orifice of the airway; the back side can be fluidly connected, via a selector-valve, with either the blower output or the inspiratory conduit, which is connected to the patient via a check-valve. The checkvalve is used to prevent reflow of expiration gases into the blower. That document also discloses a rotary directional-control valve which allows the transmission of the gas stream to be varied by rotating a spool to close or open a passage through an oblong opening.